1. Field of the Invention
The present invention generally relates to electron beam sources and more particularly to a micro-electron gun known also as micro-field emission gun and a fabrication process thereof.
2. Description of the Related Art
Micro-field emission guns have been studied originally in the purpose of breaking through the limit of operational speed of solid-state devices. In such a study, attempts have been made to fabricate integrated circuits of vacuum tubes by using the microfabrication technology developed in the art of semiconductor fabrication. Recently, however, intensive efforts are being made to construct a flat panel display by arranging such micro-field emission guns in a two-dimensional plane, such that an image is formed on a screen opposing such a field emitter array by the electron beams emitted from the micro-field emission guns forming the field emitter array. It should also be noted that such micro-field emission guns are advantageous in the point that one can produce a high energy electron beam without using bulky columns conventionally used for producing such a high energy electron beam. Thus, the possibility has now emerged to construct very compact electron microscopes or other analyzing tools that use such accelerated electron beams by using the micro-field emission guns.
FIG. 1 shows the construction of a conventional micro-field emission gun.
Referring to FIG. 1, the micro-field emission gun is constructed on a semiconductor substrate 11 such as Si and includes a sharply pointed conical emitter 12, wherein the emitter 12 is surrounded by a gate electrode 13. The gate electrode 13 induces an electric field between the gate electrode 13 and the emitter 12 such that the electrons are emitted from the emitter 12 as a result of field emission. The emitter 12 may have a diameter of 2 .mu.m and is formed in a hole 14a that is formed in an insulation film 14 of SiO.sub.2 or SiO covering the surface of the substrate 11 such that the hole 14a exposes the surface of the substrate 11. Typically, a number of such 14a are formed in rows and columns in the insulation film 14 with a pitch of about 300 .mu.m, and accordingly, the emitters 12 are also formed in rows and columns with a corresponding pitch of about 300 .mu.m. In such a construction, a large electric field is induced in response to the control voltage applied to the gate electrode 13, while such a large electric field causes a deformation in the surface potential barrier of the conductor material such as Si or W that forms the emitter 12. Thereby, the electrons are emitted to the exterior of the emitter 12 by passing through the deformed surface potential barrier by tunneling effect. The structure shown in FIG. 1 can be fabricated easily by the microfabrication technology used in the production of semiconductor devices.
FIGS. 2A-2D show the fabrication process of the micro-field emission gun of FIG. 1.
Referring to FIGS. 2A-2D, a mask pattern 12a of SiO.sub.2 is provided in the step of FIG. 2A on a part of the silicon substrate 11 on which the emitter 12 is to be formed, and a reactive ion etching process (RIE) is conducted in the step of FIG. 2B upon the substrate 11 while using the pattern 12a as a mask. Thereby, the RIE process is set such that the etching proceeds obliquely to the surface of the substrate 11, and one obtains a truncated-conical region 12b in correspondence to the mask 12a.
Next, the surface of the substrate 11 is subjected to oxidation while leaving the mask 12a such that an oxide film 12c is formed on the inclined, conical surface of the region 12b. Further, an insulation layer 14 of SiO and a layer of Cr to be used for the gate electrode 13, are deposited consecutively upon the silicon oxide film 12c on the substrate 11. Thereby, one obtains a structure shown in FIG. 2C.
Further, by removing the mask pattern 12a, a structure of FIG. 2D is obtained. In the structure of FIG. 2D, it should be noted that one can form a sharply pointed structure by removing the oxide film 12c.
In the micro-field emission gun of the structure of FIG. 1 or FIG. 2D, an acceleration voltage of several hundred volts is applied across the gate electrode 13 and the substrate 11, and an electron beam of several hundred electron volts is obtained. On the other hand, this means that an acceleration voltage of several thousand kilovolts has to be applied across the substrate 11 and the gate electrode 13 in order to obtain an accelerated, high energy electron beam of several kilo-electron volts, which are required in electron microscopes or other various analyzing tools. As the insulation layer 14 has a thickness of about 1 .mu.m or less, such an application of high acceleration voltage results in a formation of a very high electric field in the order of 10.sup.9 V/m in the insulation layer 14. Thereby, a leakage current of several micro-amperes cannot be avoided in the insulation layer 14.
In order to reduce the level of the leakage current, it is necessary to increase the thickness of the insulation layer 14 to be larger than 10 .mu.m, while the formation of such a thick insulation layer by means of conventionally used semiconductor fabrication processes such as CVD or sputtering is difficult. It is, of course, possible to bond a thick glass slab upon the gate electrode and provide an acceleration electrode upon such a glass slab by means of adhesives, while such a use of adhesives raises a problem in that the gas released from the adhesives may cause a contamination of the field emitter guns and hence undesirable deterioration of the emission characteristics thereof.